Dynamic control systems for automotive vehicles have recently begun to be offered on various products. Dynamic control systems typically control the yaw of the vehicle by controlling the braking effort at the various wheels of the vehicle. Yaw control systems typically compare the desired direction of the vehicle based upon the steering wheel angle and the direction of travel. By regulating the amount of braking at each corner of the vehicle, the desired direction of travel may be maintained. Typically, the dynamic control systems do not address roll of the vehicle. For high profile vehicles in particular, it would be desirable to control the rollover characteristic of the vehicle to maintain the vehicle position with respect to the road. That is, it is desirable to maintain contact of each of the four tires of the vehicle on the road.
In vehicle roll stability control it is desired to alter the vehicle attitude such that its motion along the roll direction is prevented from achieving a predetermined limit (rollover limit) with the aid of the actuation from the available active systems such as controllable brake system, steering system and suspension system. Although the vehicle attitude is well defined, direct measurement is usually impossible.
There are two types of vehicle attitudes needed to be distinguished. One is the so-called global attitude, which is sensed by the angular rate sensors. The other is the relative attitude, which measures the relative angular positions of the vehicle with respect to the road surface on which the vehicle is driven. The global attitude of the vehicle is relative to an earth frame (or called the inertia frame), sea level, or a flat road. It can be directly related to the three angular rate gyro sensors. While the relative attitude of the vehicle measures the relative angular positions of the vehicle with respect to the road surface, which are always of various terrains. Unlike the global attitude, there are no gyro-type sensors that can be directly related to the relative attitude. A reasonable estimate is that a successful relative attitude sensing system utilizes both the gyro-type sensors (when the road becomes flat, the relative attitude sensing system recovers the global attitude) and some other sensor signals.
One reason to distinguish relative and global attitude is due to the fact that vehicles are usually driven on a three-dimensional road surface of different terrains, not always on a flat road surface. Driving on a road surface with a large road bank does increase the rollover tendency, i.e., a large output from the global attitude sensing system might well imply an uncontrollable rollover event regardless of the flat road driving and the 3-D road driving, However driving on a three-dimensional road with moderate road bank angle, the global attitude may not be able to provide enough fidelity for a rollover event to be distinguished. Vehicular rollover happens when one side of the vehicle is lifted from the road surface with a long duration of time without returning back. If a vehicle is driven on a banked road, the global attitude sensing system will pick up certain attitude information even when the vehicle does not experience any wheel lifting (four wheels are always contacting the road surface). Hence a measure of the relative angular positions of the vehicle with respect to the portion of the road surface on which the vehicle is driven provides more fidelity than global attitude to sense the rollover event when the vehicle is driven on a road with a moderate bank angle. Such an angle is called body-to-road roll angle and it is used as one of the key variables in the roll stability control module to compute the amount of actuation needed for preventing an untripped rollover event.
When the vehicle does not have one side lifted, U.S. Pat. No. 6,556,908 does provide a method to calculate the relative attitudes and their accuracy may be affected by the vehicle loading, suspension and tire conditions. However, during a potential rollover event, such a relative roll angle is not a good measure of the true relative roll angle between vehicle body and the road surface. U.S. patent application Ser. No. 10/459,697, filed Jun. 11, 2003 provides another way to compute the true relative roll angle during a potential rollover event. This application is suited for cases where vehicle loading and suspension conditions are very close to the nominal systems. If the vehicle has large loading variations (especially roof loading), potential inaccuracy could cause false activations in roll stability controls.
During a potential rollover event, one or two wheels on the inside of the vehicle turn are up in the air and there is an angle between the axle of the lifted wheel and road surface. Such an angle is called a wheel departure angle. If such a wheel departure can be somehow characterized, the true body-to-road roll angle can be conceptually obtained as the sum of the wheel departure angle and the relative roll angle calculated in U.S. Pat. No. 6,556,908.
Another way to capture the true body-to-road roll angle is to use the resultant angle obtained by subtracting the road bank angle for the global roll angle calculated for example in U.S. Pat. No. 6,631,317. Although this method is theoretically feasible, it has inevitable drawbacks. The first drawback lies in the computation of the road bank angle, since there is no robust and accurate computation of road banks using the existing sensor set. Secondly, the global roll angle computation as shown in U.S. Pat. No. 6,631,317 may be affected by the accuracy of the low frequency bank angle estimation.
Therefore, the aforementioned two methods of computing the body-to-road roll angle may not deliver accurate enough body-to-road roll angle for roll stability control purpose in certain situations.
Various accident avoidance systems are being developed that have cameras mounted in the vehicle. Some systems also use the camera in determining when to deploy an airbag. These systems, however, do not prevent the vehicle from rolling over. Such systems merely react to a condition to reduce occupant injury.
Therefore, it is desirable in vehicle dynamics control, especially for roll stability control to detect accurately various roll angles associated with the vehicle and road to accurately predict the true roll position of the vehicle to properly activate the vehicle control systems.